Barnyard pools and sewage treatment ponds often develop a rich green bloom of algae. A superficial microscopic examination of water from such a pool usually reveals large numbers of active green cells, and a closer inspection may reveal one to several of the more than 750 species of euglenoids, of which Phacus and Euglena are common examples (Fig. 18.23Aand B).
A Euglena cell is spindle shaped, has no rigid wall, and can be seen to change shape even as the organism moves along. Just beneath the plasma membrane are fine strips that spiral around the cell parallel to one another. The strips and the plasma membrane are devoid of cellulose and together are called a pellicle. A single flagellum, with numerous tiny hairs along one side, pulls the cell through the water. A second very short flagellum is present within a reservoir at the base of the long flagellum.
Other features of Euglena include the presence of a gullet, or groove, through which food can be ingested. The food of most of the 500 Euglena species is ingested, with only about a third having several to many mostly disc-shaped chloroplasts that permit photosynthesis to take place. A red eyespot, which along with the short flagellum is associated with light detection, is located in the cytoplasm near the base of the flagella. A carbohydrate food reserve called paramylon normally is present in the form of small whitish bodies of various shapes.
Figure 18.23B A single Euglena.
Reproduction is by cell division. The cell starts to divide at the flagellar end and eventually splits lengthwise, forming two complete cells. Sexual reproduction is suspected but has never been confirmed.
Some species of Euglena can live in the dark if appropriate food and vitamins are present. Others are known to reproduce faster than their chloroplasts under certain circumstances, so that some chloroplast-free cells are formed. As long as a suitable environment is provided, these cells also can survive indefinitely. In the past, when only two kingdoms were recognized, Euglena's capacity to satisfy its energy needs through either photosynthesis or ingestion of food resulted in its being treated as a plant in botany texts and as an animal in zoology texts.
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